Many different types of muffler structures have been proposed, and are in use, for reducing the sound of the exhaust gases flowing from internal combustion engines into the atmosphere. Most of such mufflers attempt to provide such sound reduction with a minimum or back pressure to the engine. In actual practice, however, the most widely used mufflers, and even some highly specialized mufflers, tend to create a substantial amount of back pressure on the engine.
Back pressure causes significant reduction in the useful horsepower of an engine. Back pressure is created, typically, when the exhaust pulse from the engine encounters too much resistance (or not enough acceptance) when it is supplied to the muffler. As a consequence, the exhaust pulse tends to bounce or reflect at least a portion of the pulse back to the engine. Since this occurs with every outgoing pulse, reflected waves or standing waves are produced in the muffler. Since standing waves cause higher pressures on the exhaust system, the ability of the engine to exhaust gases completely from the combustion chamber is reduced. This results in reduced horsepower.
Reflected waves, sometimes known as reversion, cause significant horsepower reduction, or detrimental effects on the engine performance, particularly during the overlap period of the valve openings. Higher horsepower engines have a greater amount of valve or cam overlap, and attempts to significantly attenuate the noise from such engines results in large horsepower reduction (from 20% to 40%) directly caused by the back pressure created in the muffler. This horsepower reduction results from reflected pulses traveling back up the exhaust system to force spent gases back into the combustion chamber. When the pulses are strong enough, they can revert back into the intake manifold during the time when both intake and exhaust valves are open (the cam overlap period). When this occurs, the air-fuel mixture supplied through the intake valve is not only the desired new air and fuel, but also includes an unpredictable amount of spent gases. Primarily because of the unpredictable amounts of spent gases which are recirculated back into the intake manifold, proper ignition is difficult, particularly if large concentrations of spent gases surround the sparkplug at the time it is fired.
Also, if an exhaust system is subjected to relatively high back pressures from the muffler system, the combustion chamber cannot be emptied to a pressure less than the back pressure to which it is subjected. This is the reason an engine exhibits a significant loss of efficiency, since the new gases and fuel vapor are drawn into the cylinder on the basis of a vacuum, which starts with the exhaust pressure. The higher this pressure is, the lower the amount of air and fuel mixture which can be drawn into the cylinder for the next firing cycle.
From the above, it is apparent that any muffler attached to an engine to suppress its sound, which also causes the development of back pressure to the engine, is not satisfactory. Baffle-type mufflers have been developed for providing the desired sound attenuation. To minimize the back pressure created by baffle-type mufflers, such mufflers should be made as large as possible, either in length or diameter. This creates another problem, however inasmuch as the bulk and weight of such mufflers is unacceptable in most applications.
Two mufflers, which have been designed in effort to overcome the problems mentioned above by employing concentric cone-like baffles, are disclosed in the patents to McCurdy U.S. Pat. No. 2,214,770 and Burstein U.S. Pat. No. 4,685,534. The sound attenuator devices of both of these patents are of a generally cylindrical shape. In the devices of both patents, cone-line baffles are employed, and are concentrically arranged with the apex of one cone extending at least partially into the flared opening of the next preceding one in the series. In the devices of both patents, the exhaust gases are directed toward the apices of the cones, and then are spaced uniformly around the outer edges. In the device of the McCurdy patent, the gases are exhausted at different stages, or at different intervals along the length of the device. Consequently, this device would not be acceptable for most automotive applications.
In the device shown in the Burstein patent, the baffles are arranged for the purpose of providing or forming three concentric streams of flowing gases moving at different velocities. The problem with this patent, however, is that gases from the main, nearly straight-through path in the device, are tapped off and added to the surrounding concentric streams in a constricted region; so that back pressure relief is not provided. An examination of the structure of McCurdy also reveals constricted areas which are impacted by the exhaust gases, and which inherently produce back pressure.
Three patents to Egan U.S. Pat. No. 1,170,807; Wood U.S. Pat. No. 2,330,534; and Flugger U.S. Pat. No. 4,809,812 are directed to mufflers which split the flow of gases passing through them by means of conically shaped baffles. The purpose is to reduce back pressure from the exhaust pipe, and to provide for splitting and recombination of the gases passing through the devices to produce the desired sound reduction. Although there is a splitting and reconverging of exhaust gases passing through the devices of all three of these patents, the baffles are all symmetrical; so that all of the splitting and recombining of the pulses of exhaust gases passing through the devices is in phase as the gases pass through the baffle arrangement of the mufflers. In addition, the structures of all of these mufflers include various internal traps or back pressure producing discontinuities.
A muffler of the type known as a resonator-type muffler is disclosed in the patent to Moller U.S. Pat. No. 4,424,882. Considerable back pressure is introduced by a muffler of this type, as is readily apparent from an examination of the internal structure employed.
A different approach to an internal baffle configuration is disclosed in the patent to Malmsten U.S. Pat. No. 4,286,689. The muffler of this patent employs a series of generally conical insert baffles located throughout its length. The baffles, however, are oriented with the wide ends of the cones facing in the inlet side of the muffler. The exhaust gases then are restricted to exit through the smaller openings at the apices of each of the cones or near the apices of the cones. Consequently, the potential for the production of considerable back pressure is present in the muffler disclosed in this patent, even though it may be capable of providing sound attenuation.
It is desirable to provide a muffler for internal combustion engines and the like, which is capable of being fabricated in a relatively small size, and which produces improved sound attenuation with minimal back pressure.